The present invention relates generally to semiconductor wafer drying apparatus, and more particularly to methods of calibrating such drying apparatus.
Semiconductor wafers are generally prepared from a single-crystal ingot, such as a silicon ingot, which is sliced into individual wafers. The wafers are subjected to a number of wafer processing operations to reduce the thickness of the wafer, remove damage caused by the slicing operation, and to create a highly reflective surface.
The processing operations include cleaning the wafers in a liquid (e.g., de-ionized water) bath and then drying the wafers to inhibit formation of a residue on the surface of the wafer. The wafers may be dried in a batch drying apparatus. One type of batch drying apparatus, commonly referred to as an xe2x80x9cIPA dryerxe2x80x9d, comprises a vessel for receiving the wafers, a heater for heating a liquid solvent, e.g., isopropyl alcohol (IPA), contained in the vessel, and condensing coils for condensing the IPA vapor. The heater heats the IPA to its boiling point so that a xe2x80x9ccloudxe2x80x9d of IPA vapor is formed in the vessel. A door of the vessel is opened, and a batch of wafers is introduced to the vessel and positioned above the surface of the liquid IPA. Due to the relatively cool temperature of the wafers, and due to the door of the vessel being opened, the temperature inside the vessel drops significantly, and the cloud dissipates. After the door is again closed, and after a xe2x80x9crecovery timexe2x80x9d in which the temperature in the vessel significantly increases, the vapor cloud should re-form to envelop the wafers so that, ideally, the liquid on the surface of the wafers is completely displaced by the IPA vapor and condensed IPA, as is known. After a period of time (referred to as processing time) has elapsed, the wafers are removed from the vessel and the liquid remaining on the wafer quickly evaporates.
Conventionally, the dryer is set at a predetermined heater temperature (typically the temperature recommended by the manufacturer) during processing of successive sets of wafers, and each set of wafers is processed inside the dryer for the same processing time. The heater temperature and processing time are not typically calibrated for the particular dryer or for the mass of the wafers to be processed. For example, the vapor cloud produced in the dryer is not monitored during processing. Over time, the efficiency of the dryer""s heater may change so that less heat is delivered to the IPA by the heater. Because of the change, the xe2x80x9crecovery timexe2x80x9d, i.e., the time for the cloud of IPA vapor to form after the door is closed, may be longer than expected and the vapor cloud may not substantially envelop the wafers. If envelopment does not occur, or does not occur for a sufficient time, the DI water may not be completely displaced from the wafers. In such cases, a residue may be left on the wafer which is not detectable through industry standard post-drying inspection. Such residue can cause serious defects, such as light point defects, which will only be detectable after the wafers undergo further downstream processing such as epitaxial treatment or chemical vapor deposition. On the other hand, if the heater processing time is set too long and the wafers are exposed to the vapor cloud for too long, or temperature is set too high and the IPA is overheated, other serious defects in the wafers may also occur. In either case, the defects may result in the wafers being rejected and may thereby reduce the yield of acceptable wafers. Thus, a method of calibrating individual IPA dryers is needed.
Among the several objects and features of the present invention may be noted the provision of a method for calibrating a semiconductor wafer dryer which ensures that wafers are completely dried after processing in the dryer; the provision of such a method which does not overheat wafers during processing in the dryer; and the provision of such a method which inhibits defects in semiconductor wafers processed in the dryer.
Briefly, a method of the invention for calibrating a semiconductor wafer drying apparatus including a heater and a vessel containing a solvent and capable of receiving semiconductor wafers comprises selecting a test heater temperature and a test processing time. A first set of wafers is placed in the vessel and the heater is operated at the test heater temperature so that a solvent vapor cloud is created in the vessel. The first set of wafers is monitored for substantial envelopment by the vapor cloud during the test processing time. Based on the monitoring step, at least one of the test heater temperature and the test processing time is adjusted to establish at least one operating parameter of an operating heater temperature parameter and an operating processing time parameter for processing successive sets of wafers so as to promote substantial vapor cloud envelopment of each set of wafers dried in the drying apparatus to inhibit defects in each set of wafers.
In another aspect of the invention, the method of calibrating the semiconductor wafer drying apparatus including a vessel containing a liquid comprises selecting a test heater temperature and a test processing time and placing a first set of wafers in the vessel. The heater is operated at the test heater temperature so that a vapor cloud is created in the vessel and the presence of the vapor cloud above the first set of wafers is monitored during the test processing time. Substantial envelopment of the first set of wafers is achieved when the vapor cloud is detected above the first set of wafers. Based on the monitoring step, at least one of the test heater temperature and the test processing time is adjusted to establish at least one operating parameter of an operating heater temperature parameter and an operating processing time parameter for processing successive sets of wafers.
In yet another aspect of the invention, the vessel contains a temperature sensor positioned above the first set of wafers and the method comprises, among other things, monitoring the temperature sensor for substantial envelopment of the first set of wafers by the vapor cloud during said test processing time. Based on the monitoring step, at least one of the test heater temperature and the test processing time is adjusted to establish operating parameters including an operating heater temperature parameter and an operating processing time parameter for processing successive sets of wafers so that the successive sets of wafers are substantially enveloped by the vapor cloud for a target time segment of at least about 30 seconds.
Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.